Honda HR-V: EPS System Description - Overview

System Outline

This vehicle is equipped with electrical power steering (EPS). The driver's steering force is assisted by an electric motor at the steering column. Compared to a hydraulic assist power steering system, EPS is more efficient because it does not need an engine driven oil pump to generate hydraulic pressure.

The EPS control unit monitors and controls the EPS motor's assisting force to match driving conditions.

• Low vehicle speeds: High power assist (for easy handling)
• High speed driving: Low power assist (for stable driving)
• Low speed to high speed driving: Change smoothly from high assist to low assist

Components

The EPS system mainly consists of the following parts:

• The EPS control unit: Controls EPS motor
• The EPS motor: Generates assisting torque
• The torque sensor: Measures the steering force applied to the steering wheel
• The steering gearbox: Transmits steering force

The EPS control unit, the EPS motor, the torque sensor are combined into the steering column.

EPS SYSTEM DESCRIPTION - SYSTEM DIAGRAM

The system diagram of the EPS system is shown below.

EPS SYSTEM DESCRIPTION - TORQUE SENSOR

The torque sensor uses the Hall IC method to detect steering torque and road surface reaction torque.

The torque sensor is comprised of a middle shaft, input shaft, lower shaft, torsion bar, magnetic yoke, multi-pole magnet, and other parts.

Input from the steering wheel is transmitted to the middle shaft. The middle shaft and lower shaft are connected by the torsion bar, and when the road surface resistance is large and the lower shaft turns with difficulty, the middle shaft and lower shaft rotate differently, causing the torsion bar to twist.

But since a multi-pole magnet is connected to the middle shaft and a magnetic yoke to the lower shaft, the torque variation is detected from the amount of twisting of the torsion bar, by the multi-pole magnet, magnetic yoke, and a position control sensor.

As the torsion bar is twisted according to input from the steering wheel, a difference arises in the rotation of the middle shaft and lower shaft. A difference in rotation also arises between the multi-pole magnet and magnetic yoke teeth, and a variation is created in the N/S surface ratio of the multi-pole magnet opposite the magnetic yoke teeth. This N/S surface ratio variation magnetizes the outer edge of the magnetic yoke, and the magnetic flux of the magnetic yoke is transferred to a magnetism collecting ring. The transferred magnetic flux enters the magnetic sensor, and the difference in rotation between the multi-pole magnet and magnetic yoke teeth is output as torque variation. In addition, the temperature differential and other data in the magnetic sensor are used to correct the variation arising in the magnetic flux.

To secure reliability in the torque sensor output signal, there is a main and sub double circuit that outputs two torque signals. When no turning torque is applied to the steering wheel (neutral position), the main and sub output values are 2.5 V each. The main output value rises above 2.5 V when the steering wheel is turned to the right and drops below 2.5 V when it is turned to the left. The sub output value drops below 2.5 V when the steering wheel is turned to the right and rises above 2.5 V when it is turned to the left.

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